BEIRUT — A surge in Syrian government airstrikes has killed at least 200 people over the past two days on the outskirts of Damascus, aid agencies said Tuesday, as the Syrian war ramps up and civilians again pay the price for the failure of international efforts to resolve it.

The bloodshed marked one of the deadliest episodes in the ­seven-year war, aid agencies and human rights monitors said, as nightfall brought more waves of bombing against the cluster of towns and villages known as Eastern Ghouta, one of the largest areas still under rebel control.

Residents said they had been cowering in their basements since Sunday as squadrons of fighter jets circled over their neighborhoods, taking turns bombing. When the jets run low on fuel, they are replaced by more planes, sustaining a continuous roar of aircraft punctuated by explosions, said Firas Abdullah, an activist with the Ghouta Media Center.

Helicopters joined the warplanes, dropping crude devices known as barrel bombs crammed with nails and metal to make them more lethal, he said. “The situation here was always bad, but this is the worst we have ever seen,” Abdullah said.

Hundreds of people have been injured, flooding hospitals and clinics with casualties they are ill-equipped to treat after four years under siege. Medical facilities have been hit, too, with 12 hospitals and clinics struck by bombs and knocked out of action in the past two days, according to the Union of Medical Care and Relief Organizations (UOSSM). It put the toll of two days of bombing at more than 200 killed and 700 injured.

“The sheer intensity of airstrikes is leveling the city and killing civilians without any regard or mercy,” said Zedoun Al Zoebi, the head of UOSSM. “Medicine and medical supplies have not been allowed into the city for months now, and there is virtually no medical care available for these people as they suffer severe trauma wounds.”

The Syrian American Medical Society (SAMS), which also supports hospitals in the area, put the 48-hour death toll at 250, the highest in Syria in an equivalent time frame since a 2013 sarin gas attack in Eastern Ghouta that killed about 1,400 people.

“Hospitals are overwhelmed. Floors are overflowing with injured and blood. Those patients we discharged a couple of days ago are now back with more serious injuries,” SAMS quoted a doctor in Eastern Ghouta as saying.

The strikes are a reminder that a conflict many thought was winding down still has not run its course. Rather, many more battles lie ahead as Russian peace efforts falter and as Syrian President Bashar al-Assad seeks to assert his authority over all the areas he lost during the rebellion that erupted seven years ago.

American diplomacy is absent, hamstrung by the loss of U.S. influence since Russia intervened in support of Assad in 2015 and by a vacuum in the Syria office at the State Department. The last Syria envoy departed in March and has not been replaced.

State Department spokeswoman Heather Nauert said the United States is “deeply concerned” by the bloodshed in Eastern Ghouta. “This escalation is exacerbating the already grave human suffering of nearly 400,000 people,” she said, expressing U.S. support for a United Nations appeal for a month-long cease-fire that was made more than two weeks ago.

The latest surge of violence is part of what appears to be a fresh government push to recapture the Eastern Ghouta area, which has been under the control of rebels since 2012 and besieged by government forces since 2013. Spanning 142 square miles, it is populated by more than 350,000 people.

Over the weekend, pro-government social media accounts posted photographs of columns of armored vehicles and trucks carrying fighters from the renowned loyalist Tiger militia and converging at the front lines around Eastern Ghouta, triggering speculation that an offensive was imminent.

The airstrikes began with a vengeance overnight Sunday, sending residents scurrying for cover. Opposition activists and journalists began posting photographs of the victims, many of them children. One showed the bodies of four children laid out in white shrouds, splashed with red blood and flanked by the bodies of three adults — apparently a family that had died together. Another photograph, circulated by activists Monday, appeared to show the bodies of five members of a family, the children wrapped in their dressing gowns, one of them cuddling a dead woman who may have been their mother.

“What’s the goal? Is it to crush Ghouta on the heads of everyone like they crushed Aleppo?” asked Osama Nasser, a veteran anti-government activist who said he and his family had not left their basement since Sunday.

The rebels have hit back, firing more than 45 mortar shells and rockets into Damascus, killing six people and injuring 29, according to the Syrian state news agency SANA.

The Syrian government sent a letter to the United Nations accusing Western nations of responsibility for the attacks, because of the support that Western governments have given to rebels in the past, the agency said. The Syrian government urged the United Nations to condemn Western governments “as they are denying the Syrian state’s right to defend its citizens, fight terrorism, and confront those who fund and arm terrorists,” SANA said.

The United Nations repeated its appeal for a cease-fire on all sides, focusing its call on halting government attacks against Eastern Ghouta.

“The humanitarian situation of civilians in East Ghouta is spiraling out of control,” the U.N. regional humanitarian coordinator for Syria, Panos Moumtzis, said in a statement. “It’s imperative to end this senseless human suffering now. Such targeting of innocent civilians and infrastructure must stop now.”

The U.N. children’s agency, UNICEF, issued a statement that consisted of a blank page. “We no longer have the words to describe children’s suffering and our outrage,” the agency added in a footnote, by way of explanation.

Western aid agencies, which have been unable to operate inside Syria for years because of government restrictions but have partner arrangements with local aid agencies, echoed the calls for a cease-fire.

“The extreme escalation in violence has made it impossible for humanitarian agencies to reach the hundreds of thousands of vulnerable civilians trapped in Eastern Ghouta. Civilians are deprived of food and medicine and are facing hunger and death. If a cease-fire is not reached now, we will be facing a humanitarian catastrophe,” said Wouter Schaap, CARE’s country director for Syria.

The area is controlled by an assortment of mostly Islamist rebel groups, the biggest of which, Jaish al-Islam, has participated in a year-old Russian-led initiative to resolve the conflict through a series of cease-fires. Eastern Ghouta was designated as one of the “de-escalation” zones that formed the backbone of the process and were intended to diminish levels of violence to allow a chance for negotiations.

But a breakdown of the cease-fires in Eastern Ghouta and other parts of the country in recent weeks appears to spell an end to Russia’s efforts to resolve the conflict.

“All the talk about a cease-fire, about cutting a deal between opposition and regime, all of it was lies,” said Nasser, the opposition activist. “The regime wants to be the only one that will rule this country, by fire and by iron.”